Objective  The drilling wastewater in a block of Changqing Oilfield is large in volume, complex in composition, and high in Ca²⁺ and other multivalent ions, which makes it challenging to meet the requirements of fracking fluid blending performance after conventional treatment.

Methods  Based on the above difficult process characteristics, a new type of calcium removal process was proposed by modifying chitosan with multifunctional groups to chelate Ca²⁺, and its effect on calcium removal, calcium removal mechanism, applicable conditions (temperature, pH), environmental impact (mud production) and back-mixing performance were evaluated.

Results  The results showed that chitosan with a high degree of deacetylation was used as raw material, potassium persulfate was used as initiator to graft acrylic acid, and chitosan: potassium persulfate: acrylic acid=1.0:1.2:25.0 was reacted under the conditions of temperature of 70 ℃ for 2 h to produce highly water-soluble multifunctional chelating agent, and the Ca²⁺ could be chelated and removed to less than 500 mg/L by using 600 mg/L SZ-1. SZ-1 could be applied under the conditions of 10-40 ℃, pH=6-14, with low sludge production (0.016 g/L) and suitable viscosity of base fluid (30.9~31.2 mPa∙s), which satisfied the requirements of base fluid viscosity range (10-40 mPa∙s) in SY/T 7627—2021 Technical requirements of water-based fracturing fluid.

Conclusion  It is expected to provide a new solution for the treatment of drilling wastewater with high calcium content in oil and gas fields, with its green and clean color, excellent performance, and simple preparation.